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Figure 41: tSRF Definition

Example A

CKE

Example B

CKE

Example C

CKE

Example D

CKE

tSRF

tREFW

tSRF2

tSRF1

tSRF

Exit self refresh mode

Enter self refresh mode

Exit self refresh mode

Enter self refresh mode

Exit self refresh mode

Enter self refresh mode

Exit self refresh mode

Enter self refresh mode

Exit self refresh mode

tREFW

Notes:

1. Time in self refresh mode is fully enclosed in the refresh window (tREFW).

2. At self refresh entry.

3. At self refresh exit.

4. Several intervals in self refresh during one tREFW interval. In this example, tSRF = tSRF1 +

tSRF2.

The mobile LPDDR2 device provides significant flexibility in scheduling a REFRESH

command as long as the required boundary conditions are met (see the tSRF Definition

figure).

In the most straightforward implementations, a REFRESH command should be sched-

uled every tREFI. In this case, self refresh can be entered at any time.

Users may choose to deviate from this regular refresh pattern, for instance, to enable a

period in which no refresh is required. As an example, using a 1Gb LPDDR2 device, the

user can choose to issue a refresh burst of 4096 REFRESH commands at the maximum

supported rate (limited by tREFBW), followed by an extended period without issuing

any REFRESH commands, until the refresh window is complete. The maximum suppor-

ted time without REFRESH commands is calculated as follows: tREFW - (R/8) × tREFBW

= tREFW - R × 4 × tRFCab.

For example, a 1Gb device at TC ≤ 85˚C can be operated without a refresh for up to 32ms

- 4096 × 4 × 130ns ≈ 30ms.

Both the regular and the burst/pause patterns can satisfy refresh requirements if they

are repeated in every 32ms window. It is critical to satisfy the refresh requirement in

216-Ball and 220-Ball, Dual-Channel LPDDR2 SDRAM

REFRESH Command

09005aef85eb530a

216b_220b_2ch_2e0e_embedded_lpddr2.pdf – Rev. F 08 /16 EN

Micron Technology, Inc. reserves the right to change products or specifications without notice.

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